Method and system for fabricating fiducials using selective area growth

ABSTRACT

A method of forming alignment marks includes providing a III-V compound substrate having a device region and an alignment mark region, forming a hardmask layer having a first set of openings on the alignment mark region exposing a first surface portion of the III-V compound substrate and a second set of openings on the device region exposing a second surface portion of the III-V compound substrate, etching the exposed surface of the III-V compound substrate using the hardmask layer as a mask to form a plurality of trenches, and epitaxially regrowing a semiconductor layer in the trenches to form the alignment marks extending to a predetermined height over the processing surface of the III-V compound substrate.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalPatent Application No. 62/875,443, filed on Jul. 17, 2019, entitled“Method and System for Fabricating Fiducials Using Selective AreaGrowth,” the disclosure of which is hereby incorporated by reference inits entirety for all purposes.

BACKGROUND OF THE INVENTION

A sequence of photolithography steps are generally required in themanufacture of semiconductor devices. Accordingly, in order to providefor alignment between a previously processed layer and a current layer,one or more fiducials (also referred to as alignment marks) can beformed as topographical steps or recesses in the processed layer. Aprotective layer (e.g., silicon oxide) may be formed over thetopographical features in order to prevent damage to the fiducials(alignment marks) in a subsequent process. However, the protective layercan limit the resolution of the fiducials and increase the cost andprocessing time.

For example, some semiconductor fabrication processes require fiducials(alignment marks) that require at least one and sometimes two or moreadditional masking steps. The additional fiducials (alignment masks) andassociated processing add tooling costs, cycle time, and potentialdefects to the process flow. Thus, there is a need in the art forimproved methods and systems related to fiducials.

SUMMARY OF THE INVENTION

The present invention relates to semiconductor manufacturing technology,and more particularly, to a method of forming alignment marks andsemiconductor devices having such alignment marks. In particular, insome semiconductor manufacturing processes, the surface of thesemiconductor can be made substantially planar at certain steps in theprocess. One such process is selective area regrowth to formsemiconductor layers embedded within an existing pattern. An example ofsuch a regrowth which provides a substantially planar surface is givenin U.S. Pat. No. 9,117,839 (Kizilyalli, et al.). In such a selectivearea regrowth process, it is desirable to be able to simultaneously formalignment structures having topographic relief relative to the planarregrowth surface.

In one aspect of the present disclosure, a method of forming alignmentmarks includes providing a III-V compound substrate having a deviceregion and an alignment mark region, forming a hardmask layer having afirst set of openings on the alignment mark region exposing a firstsurface portion of the III-V compound substrate and a second set ofopenings on the device region exposing a second surface portion of theIII-V compound substrate, etching the exposed surface of the III-Vcompound substrate using the hardmask layer as a mask to form aplurality of trenches, and epitaxially regrowing a semiconductor layerin the trenches to form the alignment marks extending to a predeterminedheight over the processing surface of the III-V compound substrate.

In another aspect of the present disclosure, a semiconductor device mayinclude a III-V compound substrate comprising a device region and analignment mark region, and a plurality of three-dimensional andthree-directional alignment marks in the alignment mark region. Each ofthe alignment marks may include a protrusion portion extending over asurface of the III-V compound substrate. In one embodiment, the III-Vcompound substrate includes an N—GaN epitaxial layer, and the alignmentmarks includes a p-GaN epitaxial layer.

Numerous benefits are achieved by way of the present invention overconventional techniques. For example, embodiments of the presentinvention utilize the enhanced growth rate of metal-organic chemicalvapor deposition (MOCVD) epitaxial films adjacent to large areas ofhardmask to create elevated features during a p-GaN regrowth process,even when the regrowth in the device array itself is planarized to thelevel of the fin top surface. Embodiments of the present invention cancreate alignment structures using a first selective area growthdefinition mask, which will be then be visible to a deep UV (DUV)aligner at a next alignment step after a p-GaN regrowth. In oneembodiment, the selective area regrowth definition mask is the sourcedefinition mask that is also the first mask in the process flowaccording to embodiments of the present invention. According to someembodiments, other types of alignment marks (e.g., i-line alignmentmarks) can be subsequently created at a subsequent metal mask patterningstep for subsequent mix-and-match alignment.

Moreover, embodiments of the present invention provide methods andsystems that concurrently generate alignment features (marks) duringepitaxial regrowth of gate regions for a vertical fin-based FET device,potentially avoiding the use additional mask layers. The novel approachutilizes the enhanced growth rate of MOCVD epitaxial films adjacent tolarge areas of a hardmask to create elevated features during the p-GaNregrowth step, even when the regrowth in the device array is planarizedto the level of the fin top surface. In some embodiments, the regrowthof the alignment features is self-limiting so that the height of thealignment features (marks) can be predetermined by an opening width of apatterned mask layer. These and other embodiments of the invention alongwith many of its advantages and features are described in more detail inconjunction with the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating measured and simulatedgrowth-rate-enhancement (GRE) profiles of GaN adjacent to differenthardmask widths according to embodiments of the present disclosure.

FIG. 2A is a top plan view of a partially patterned hardmask layerincluding an alignment mask layout and a fin layout according to anembodiment of the present disclosure.

FIG. 2B is a partial cross-sectional view of the patterned hardmasklayer taken along line A-A of FIG. 2A.

FIG. 2C is a top plan view of a p-GaN regrowth in trenches according toan embodiment of the present disclosure.

FIG. 2D is a cross-sectional view of p-GaN regrowth in trenches takenalong line B-B of FIG. 2C and hardmask removal according to anembodiment of the present disclosure.

FIG. 3 is a simplified flowchart illustrating a method of formingfiducials (alignment marks) during the formation of integrated circuitson semiconductor wafers according to some embodiments of the presentdisclosure.

FIG. 4 is an FIB cross-section showing the different growths on the0.7/3 array for a 400 nm-regrowth wafer according to an embodiment ofthe present disclosure.

FIG. 5A is an oblique view SEM image of a 0.7/3 array on a 600nm-regrowth wafer according to an embodiment of the present disclosure.

FIG. 5B is an enlarged portion of the ends of the regrown regions inFIG. 6B.

FIG. 6 is an FIB cross-section of a 0.7/3 array on a 800 nm-regrowthwafer according to an embodiment of the present disclosure.

FIGS. 7A to 7F show results of regrowth profiles on respective 200 nm,400 nm, 500 nm, 550 nm, 600 nm, and 650 nm regrowth wafers according toan embodiment of the present disclosure.

FIG. 8 is an oblique view SEM image of a 0.3 μm×25 μm long fin arraywith 2 μm pitch on a 500 nm-regrowth wafer according to an embodiment ofthe present disclosure.

FIG. 9 shows PECVD nitride hardmask sidewall angles of a 0.3 μm hardmaskwidth for a 0.3 μm×25 μm long fin array with 2 μm pitch according to anembodiment of the present disclosure.

FIG. 10 shows a top view of alignment mark groups designed to formfiducials above a hardmask when fin arrays are planar according to anembodiment of the present disclosure.

FIG. 11A shows a top view of alignment mark groups that may be used asalignment mark patterns according to some embodiments of the presentdisclosure.

FIG. 11B shows a portion of the alignment mask results on a 500 nmregrowth wafer according to an embodiment of the present disclosure.

FIG. 12A shows a top view of a hexagonal annulus pattern having aregrowth above the hardmask according to an embodiment of the presentdisclosure.

FIG. 12B shows an enlarged portion of the hexagonal annulus pattern ofFIG. 14A.

FIG. 12C is an oblique view SEM image of an enlarged portion of thehexagonal annulus pattern of FIG. 14A according to an embodiment of thepresent disclosure.

FIG. 13A is an array of hexagonal-pyramid shaped protrusions showing aregrowth above the hardmask according to an embodiment of the presentdisclosure.

FIG. 13B shows an enlarged portion of the array in FIG. 13A.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Some semiconductor devices include one or more layers of epitaxiallygrown semiconductor materials. Selective area metal-organic vapor phaseepitaxy (SA-MOVPE) is one of the fabrication methods for vertical finfield effect transistor (FinFET) devices.

FIG. 1 is a graph illustrating measured and simulated growth rateenhancement (GRE) profiles of GaN adjacent to different mask widths thatmay be utilized in embodiments of the present disclosure. FIG. 1 isdescribed in the publication “Selective Area Metal-Organic Vapor PhaseEpitaxy of Nitride Semiconductors for Multicolor Emission” by TomonariShioda et al. IEEE Journal of Selected Topics in Quantum Electronics,2009. The width of mask pairs are increased step by step. This paper ishereby incorporated by reference in its entirety. The measured value ofGRE is derived by normalizing a thickness at a given position by thethickness at the planar position. Referring to FIG. 1, the mask openingis 60 μm and the mask widths of the mask pairs vary from 30 μm atposition 1 (position is shown in μm on the x-axis) to 240 μm at position8 with an incremental amount of 30 μm at each subsequent position. Thegrowth temperature was 1150° C. and the pressure was 100 mbar with H₂carrier gas. The solid lines denote experimental (i.e., measured) GREvalues along the different mask widths of mask pairs (indicated byarrows 1 to 8) in the schematic of the mask pattern, and the dashedlines denote calculated (i.e., simulated) GRE profiles with D/k_(s)=50μm, where D is the mass diffusivity (m²/s) in the vapor phase and k_(s)is the surface reaction rate constant (m/s). Selective-area MOCVD growthof GaN is affected by the selective-growth hardmask. Gallium thatreaches the hardmask surface prefers to diffuse to the nearby GaNsurface, where it is incorporated into the growing film. As a result,the growth rate of GaN next to large-area hardmask regions is enhancedby the flux of Ga species from the hardmask surface.

A method of manufacturing a semiconductor device is described withreference to FIGS. 2A, 2B, 2C, and 2D. FIG. 2A is a top plan view of apartially patterned hardmask layer including an alignment mask layoutand a semiconductor active device area fin layout according to anembodiment of the present disclosure. The method of manufacturing asemiconductor device may include providing a semiconductor substrate,e.g., a III-nitride compound substrate. In an embodiment, theIII-nitride compound substrate is an n-type GaN substrate on which isgrown an n-type GaN epitaxial layer, a second n-type GaN epitaxial layerwith a graded dopant concentration, and a third n-type GaN epitaxiallayer with a higher dopant concentration than the first epitaxial layer.The method also includes forming a patterned hardmask layer on thesemiconductor substrate. As used herein, the terms “fiducials” and“alignment marks” are interchangeably used. The terms “alignmentfiducial” and “alignment mark” are interchangeably used. The patternedhardmask layer has an alignment mark region including a plurality ofalignment mark openings 203 and a device region including a plurality offins protected by the hardmask separated by openings 205, as shown inFIG. 2A. In some embodiments, the plurality of alignment mark openings203 each have a width of about 0.7 μm, and the pitch measured fromcenter-to-center of two adjacent alignment mark openings is about 3 μm.In some embodiments, the plurality of alignment mark openings 203 eachhave a width between 0.6 and 2.0 μm, and the pitch measured fromcenter-to-center of two adjacent alignment mark openings is about 8 μm.In some embodiments, the plurality of fin openings 205 each have a widthof about 1.8 μm, the fins each have a width of about 0.2 μm, and thepitch measured from center-to-center of two adjacent fin openings isabout 2.0 μm. Other embodiments can utilize other values for the widthand center-to-center spacing of the alignment mark openings as well asthe width and center-to-center spacing of the fin openings and thesevalues are merely exemplary. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

Referring to FIG. 2B, a hardmask layer 201 is formed on a semiconductorsubstrate 200. The hardmask layer can include one or more dielectricssuch as oxides and nitrides or other suitable materials (including,e.g., refractory metals, metal alloys or metal compounds) that can bedeposited, patterned, withstand etching processes, and withstand hightemperature regrowth processes. The hardmask has a main lateral surfacecoinciding with the (0001) plane of the semiconductor substrate. In someembodiment, the semiconductor substrate may be misoriented from the<0001> direction by an angle. The method also includes removing a firstportion in the alignment mark region and a second portion in the deviceregion of the semiconductor substrate, using the patterned hardmasklayer as a mask, by etching to form a first set of alignment masktrenches 207 in the alignment mask region and a second set of fintrenches 209 in the device region. In one embodiment, the trenches eachmay have a depth of 0.8 μm. In order to have uniform depth of thetrenches, good controllability of the etch process is required. In someembodiments, an etch process may include chlorine-based chemistry usingRIE to form the trenches, and a cleaning process may be carried outusing a TMAH solution after etching the semiconductor substrate. Thetrenches includes sidewalls that are substantially perpendicular to themain surface of the semiconductor substrate. In some embodiments, thetrench sidewalls in the device region are substantially parallel to theGaN m-plane.

The method also includes epitaxially regrowing a semiconductor layer inthe trenches until the semiconductor layer is substantially coplanarwith the base of the hardmask in the device region. At the same time, aprotrusion portion of the semiconductor layer extending over an uppersurface of the semiconductor substrate reaches a predetermined height.FIG. 2C is a top plan view of a p-GaN regrowth in the trenches accordingto an embodiment of the present disclosure. In one embodiment, the p-GaNregrowth may be a metalorganic vapor phase epitaxy (MOVPE) that allowsgood in-plane control of thickness, strain, and bandgap energy.

FIG. 2D is a cross-sectional view of the p-GaN regrowth in the trenchestaken along the line B-B of FIG. 2C and the removal of the hardmasklayer after an epitaxially regrowth of a semiconductor layer (e.g.,p-GaN) according to an embodiment of the present disclosure. Referringto FIG. 2D, the epitaxial regrowth of a semiconductor layer in thealignment mark trenches has a portion 211 protruding over the hardmaskwhile the regrowth of the semiconductor layer in the fin openings has anupper surface 213 below the hardmask, and the upper surface 213 of thesemiconductor layer is substantially planar. The dotted line denotes thebottom of the trenches 209, which is now covered by the regrowth of thesemiconductor layer. As discussed in relation to FIG. 1, because thewidth of the alignment mark openings is smaller than the width of thefin openings, the MOCVD growth rate in the alignment mark openings ishigher than the MOCVD growth rate in the fin openings as a result ofGRE. It should be noted that the growth of the semiconductor layer inthe alignment mark openings can be self-limiting when the protrudingportion over the hardmask is characterized by a triangular shape. In oneembodiment, the plane of the sidewall of the alignment mark opening issubstantially parallel to the GaN m-plane, and the protruding portion211 over the hardmask has a height that is equal about the square rootof 3 divided by 2 times the alignment mark opening width Wtr((√3/2)×Wtr). In one embodiment, a smooth surface of the regrowth facetscan be obtained at a growth temperature in a range between 800° C. and1150° C. and under a pressure of about 100 mbar with H₂ carrier gas. Inone embodiment, the growth temperature is in a range between 850° C. and1100° C., preferably between 900° C. and 1050° C., and more preferablybetween about 930° C. and 970° C., e.g., 950° C. in an embodiment. Thus,embodiments of the present invention utilize masks with two-dimensionalshapes to grow three-dimensional alignment marks. The growth surfaces ofthe three-dimensional alignment marks (i.e., the regrowth facets shownin FIG. 2D) are bounded by crystallographic planes that producealignment marks that can be referred to as multi-directional (e.g., sixdirectional). As an example, the growth surfaces can be aligned parallelto the m-planes of the substrate, i.e., the <1000>, <0010> and <0100>axes.

FIG. 3 is a simplified flowchart illustrating a method 300 of formingfiducials (alignment marks) during the formation of integrated circuitson semiconductor wafers according to some embodiments of the presentdisclosure. Method 300 may include providing a group III-V compoundsubstrate having a device region (also referred to as an active region)and an alignment mark region (301). In one embodiment, the substrate mayinclude a GaN material. Method 300 may further include forming apatterned hardmask layer on the group III-V compound substrate. Thepatterned hardmask layer includes a first set of openings on thealignment region and a second set of openings on the device region(block 303). In one embodiment, a hardmask layer includes siliconnitride and may be formed on the substrate by a PECVD or a LPCVDprocess. The hardmask layer is then patterned by lithography thatincludes forming a patterned photoresist on the hardmask layer and thenetching the hardmask layer to form a patterned hardmask layer. In oneembodiment, the patterned hardmask layer has a first set of openings onthe alignment mark region exposing a first surface portion of thesubstrate and a second set of openings on the device region exposing asecond surface portion of the substrate, as shown in FIG. 2A. In oneembodiment, each of the first set of openings has a width of about 0.7μm and a length of about 35 μm. The pitch of two adjacent openings ofthe first set of openings is about 3 μm. The first set of openings isalso referred to as a 0.7/3 array herein, i.e., the opening width of the0.7/3 array is about 0.7 μm, and the pitch of the 0.7/3 array is about 3μm. In one embodiment, each of the second set of openings has a width ofabout 1.8 μm and a length that can be the same or different from thelength of the first set of openings. The pitch of two adjacent openingsof the second openings is about 2.0 μm. The second set of openings isalso referred to as a 1.8/2.0 array, i.e., the width and the pitch ofthe 1.8/2.0 array are 1.8 μm and 2.0 μm, respectively. Other embodimentscan utilize other values for the width and center-to-center spacing ofthe openings and these values are merely exemplary. One of ordinaryskill in the art would recognize many variations, modifications, andalternatives.

Method 300 further includes etching the exposed first surface portionand the second surface portion of the substrate to form a plurality oftrenches (305). In one embodiment, the trenches each has a depth ofabout 0.8 μm. Method 300 also includes epitaxially regrowing asemiconductor layer in the trenches (307) until a portion of thesemiconductor layer in the second portion of the substrate becomessubstantially planar to the lower surface of the hardmask layer, asshown in FIG. 2D. Simultaneously, a protrusion portion of thesemiconductor layer in the first portion of the substrate reaches apredetermined height above the upper surface of the hardmask layer. Thepredetermined height may be determined by the width of the trenches. Inone embodiment, the protrusion portion may be self-limited and have ashape of an isosceles triangle or a trapezoid with a base that issubstantially flush with the substrate. For example, the protrusionportion can have a height that is equal the product of the width of thebase time (sqrt(3)/2), as shown in FIG. 2D. As explained in FIG. 1,because the width (e.g., 0.7 μm) of the first set of openings is smallerthan the width (e.g., 1.8 μm) of the second set of openings, and thehardmask width (e.g., 3−0.7=2.3 μm) of the first set of openings islarger than the hardmask width (e.g., 2.0−1.8=0.2 μm) of the second setof openings, the regrowth rate of the portion of the semiconductor layerin the first set of trenches is greater than the regrowth of the portionof the semiconductor layer in the second set of trenches, as shown inFIG. 2D. Method 300 may further include removing the hardmask after theepitaxially regrowing process.

In some embodiments, the epitaxial regrowth of the semiconductor layerin the device region may include regrowing a GaN layer and implantingp-type ions (e.g., magnesium). In one embodiment, implanting p-type ionsmay be performed concurrently with the regrowth process. In anotherembodiment, implanting p-type ions may be performed after the regrowthprocess. In some embodiments, the p-type GaN is doped with Mg with adopant concentration of about 1×10¹⁹ atoms/cm³.

FIG. 4 is a focused ion beam (FIB) cross-section showing the differentgrowths on the 0.7/3 array (alignment mask array) and the 1.8/2.0 finarray (insert 401) for a 400 nm-regrowth wafer according to anembodiment of the present disclosure. Referring to FIG. 4, all trenchopenings in the 0.7/3 array show growth above the level of the hardmask,while the fin array shows non-planar growth with the growth surface(valleys) below the hardmask for a 400 nm-regrowth wafer (see insert401). As shown in the insert 401, the regrowth semiconductor layer showsa valley in the large trench (large trench width and small hardmaskwidth) between two adjacent fins in the active device region whereas theovergrowth in the alignment mask region (small trench width and largehardmask width) is almost complete. It is expected that theself-limiting alignment marks in the alignment mark region reach thepredetermined height before the regrowth of the semiconductor layer hasa planarized surface (the valley between two adjacent fins is completelyfilled).

FIG. 5A is an oblique view SEM image of a 0.7/3 array on a 600nm-regrowth wafer according to an embodiment of the present disclosure.Referring to FIG. 5A, the p-GaN regions (light stripes) are regrownabove the hardmask (dark areas). The different contrast betweendifferent p-GaN regions is caused by the different top surfaces. Thehardmask (dark area) corresponds to the hardmask layer disposed on thesurface of the substrate, and the light stripes between the hardmaskcorrespond to the protrusion portion 211 extending over the surface ofthe hardmask, as shown in FIG. 2D. the regrowth extends above thehardmask and forms triangles.

FIG. 5B is an enlarged portion of the ends of the regrown regions shownin FIG. 5A. Referring to FIG. 5B, the triangular shape of the regrowth(indicated by a dotted ellipse) extends above the hardmask (flatsurface). As discussed above in connection with FIG. 2D, the epitaxiallygrown GaN layer in the trenches has a protrusion portion extending overthe surface of the hardmask. The protrusion portion has a triangularshape that allows for precise control of the height of the protrusionportion. Referring back to FIG. 2D, when the regrowth facets meet at thevertex of the triangle, the regrowth process will stop. In other words,the regrowth of the GaN layer is self-limiting and substantially stopswhen the height or thickness of the protrusion portion extending overthe surface of the hardmask reaches a predetermined value, which isdetermined by the width of the trench opening. In one embodiment, theprotruding portion over the hardmask has a height that is equal aboutthe square root of 3 divided by 2 times the alignment mark openingwidth. In one embodiment, a smooth surface of the regrowth facets can beobtained at a growth temperature in a range between 850° C. and 1100°C., preferably between 900° C. and 1050° C., and more preferably betweenabout 930° C. and 970° C., and under a pressure of in a range of about100 mbar to about 800 mbar with H₂ carrier gas. Therefore, according tosome embodiments of the present disclosure, alignment features (marks,fiducials) can be formed by forming protruding members on a III-nitridesubstrate. The three-dimensional (3D) nature of the protrusion of thealignment features improve the viewability relative to the planar deviceregion and allow alignment in the x and y directions.

FIG. 6 is an FIB cross-section of a 0.7/3 array on a 800 nm-regrowthwafer according to an embodiment of the present disclosure. Referring toFIG. 6, the regrowth of the alignment marks in the trench has atriangular-shaped portion 701 that extends above the hardmask. Thealignment marks are self-limiting as will be described below withreference to FIGS. 7A through 7F, which show that the regrowth layerbetween adjacent fins has a planarized (flat) surface at the 650 nmregrowth. It should be noted that layer 603 is a protective layer thatis needed only to protect the structure for the FIB process. The layer603 is not present in the normal process, it is needed as a result ofthe preparative measure for the FIB procedure.

FIG. 7A is an SEM image showing a vertical regrowth profile of asemiconductor layer in a recess between adjacent semiconductor fins on a200 nm regrowth substrate in a 1.8/2.0 fin array. Voids 710 are presentunder the lateral portion at the 200 nm regrowth process. These voids710 are in a protective layer that is needed only to protect thestructure for the FIB process. The voids 710 and the protective layerare not present in the normal process, and occur only as a result of thepreparative measure for the FIB procedure.

FIG. 7B is an SEM image showing a vertical regrowth profile of asemiconductor layer in a trench between adjacent semiconductor fins on a400 nm regrowth substrate in a 1.8/2.0 fin array. For this 400 nmregrowth, a valley 712 between adjacent semiconductor fins is present.It is noted that a layer 701 is present over the hardmask. Layer 701 isa protective layer that is needed only to protect the structure for theFIB process. Layer 701 is not present in the regrowth process, it isonly needed as a result of the preparative measure for the FIBprocedure.

FIG. 7C is an SEM image showing a vertical regrowth profile of asemiconductor layer in a trench between adjacent semiconductor fins on a500 nm regrowth substrate in a 1.8/2.0 fin array. Divots 714 betweenadjacent semiconductor fins are present.

FIG. 7D is an SEM image showing a vertical regrowth profile of asemiconductor layer in a trench between adjacent semiconductor fins on a550 nm regrowth substrate in a 1.8/2.0 fin array. Divots 718 betweenadjacent semiconductor fins still can be seen. Comparing with divots 714in FIG. 8C, divots 716 are less deep. It can also be seen that the depthof divots 714, 716 shown in FIGS. 7C and 7D is smaller in the lateraland vertical directions than the valleys 712 shown in FIG. 7B. Theregrowth layer between the fins on the 550 nm regrowth substrate almostachieve a planar surface. Layer 701 is a protective layer for the FIBprocessing. The artifact layer 701 is not required according toembodiments of the present disclosure.

FIG. 7E is an SEM image showing a vertical regrowth profile of asemiconductor layer in a recess between adjacent semiconductor fins on a600 nm regrowth substrate in a 1.8/2.0 fin array. The upper surface ofthe regrown semiconductor layer is substantially planar and flush withthe bottom surface of the hardmask.

FIG. 7F is an SEM image showing a vertical regrowth profile of asemiconductor layer in a trench between adjacent semiconductor fins on a650 nm regrowth substrate in a 1.8/2.0 fin array. The upper surface ofthe regrown semiconductor layer is substantially planar and above thebottom surface of the hardmask. Therefore, the inventors believe that aregrowth thickness between 550 nm and 650 nm, for the initial etchedtrench depth in this example, can provide acceptable planar surface forthe semiconductor layer. Of course, one of ordinary skill in the artwould recognize many variations, modifications, and alternatives. In oneembodiment, the regrowth thickness in the active device region dependson the etch depth and the width ratio between the hardmask and thetrench. In another embodiment, the flatness of the regrowth of thesemiconductor layer in the active device region depends on the trenchdepth and the ratio between the fin width and the trench width. In oneembodiment, the active device region has a trench depth of about 0.8 μm,the trench width of about 1.8 μm, and the hardmask width of about 0.2 μm(i.e., the fin width is about 0.2 μm), so that the width ratio betweenthe trench and the hardmask in the active device region is 0.9 (1.8/0.2)to obtain a planar surface of the regrowth semiconductor layer at aregrowth thickness between 550 nm and 650 nm.

According to embodiments of the present disclosure, the hardmaskmaterial may include silicon dioxide, silicon nitride, aluminum nitride,aluminum oxide, titanium nitride, hafnium oxide, combinations thereof,or the like. The thickness of the hardmask material can vary from a fewnanometers to about four hundred nm. Conventional photolithographicsemiconductor processes, e.g., spin coating a photoresist on thehardmask material, selectively exposing the photoresist to light,developing the photoresist, and etching the hardmask material using thephotoresist as a mask. In one embodiment, the hardmask material can bedeposited by PECVD. In one embodiment, the hardmask material can bedeposited by LPCVD.

FIG. 8 is an oblique view SEM image showing a 0.3 μm wide×25 μm long finarray having a 2 μm pitch (0.3/2 array) on a 500 nm-regrowth waferaccording to an embodiment of the present disclosure. Referring to FIG.8, the epitaxial regrowth of the semiconductor layer (e.g., a gatelayer) between the fins has a substantially planar surface and nonon-uniformity. In other words, an array of fins surrounded by a gatelayer having a substantially planar surface can be formed in therecesses.

FIG. 9 is an SEM image showing PECVD nitride hardmask sidewall angles ofa 0.3 μm hardmask width for a 0.3 μm×25 μm long fin array with 2 μmpitch (0.3/2 array) according to an embodiment of the presentdisclosure. Referring to FIG. 9, the nitride hardmask layer has athickness of about 249.7 μm and an angle in a range between 78 degreesand about 82 degrees with the hardmask width of 0.3 μm. In oneembodiment, the average hardmask angle is about 80 degrees. In contrast,the angle of the vertical sidewall of the nitride hardmask by LPCVD (notshown) is about 83 degrees and higher. In other words, the sidewallerosion of the PECVD nitride hardmask is higher than that of the LPCVDnitride hardmask so that the sidewalls of the PECVD nitride hardmask areless perpendicular to the lateral surface of the substrate than thesidewall of the LPCVD nitride hardmask. The inventors concluded that theLPCVD process is superior to the PECVD process.

FIG. 10 shows a top plan view of multiple alignment mark groups havingdifferent geometries designed to create topographical regrown featuresprotruding above a hardmask when device fin arrays are substantiallyplanar to the bottom of the hardmask according to an embodiment of thepresent disclosure. Referring to FIG. 10, a plurality of alignment markgroups can be formed at the source metal mask patterning step (e.g., thefirst step in the process flow of forming a vertical FinFET device). Thedifferent alignment mark groups can include multiple alignment marks forforming stripe-shaped trenches in the alignment mark region along adirection of a plurality of fins in the device region, in a directiondifferent than a direction of the fins in the device region, andmultiple alignment marks for forming polygonal shapes in the alignmentmark region. For example, the upper portion 1011 includes alignment markgroups having stripes having different opening sizes and arranged inparallel to each other and spaced apart from each other by differenthardmask widths. The lower portion 1-12 includes alignment mark groupshaving polygonal shapes (e.g., square, rectangular, honey-comb,hexagonal, etc.) and different sizes.

FIG. 11, which includes FIGS. 11A and 11B, shows a top view of alignmentmark groups having different geometries that may be used as alignmentmark patterns of a hardmask according to some embodiments of the presentdisclosure. Referring to FIG. 11A, an upper portion of the alignmentmark groups includes a plurality of stripes having different geometriesthat are arranged in the vertical direction and in the horizontaldirection. The stripes have different widths and are spaced apart fromeach other by different distances. The lower portion of alignment markgroups includes discrete small geometries that can be square orhexagonal in a transparent portion 1101, in a semitransparent(translucent) portion 1102, or in an opaque portion 1103. FIG. 11B showsalignment mask results on a 500 nm regrowth wafer according to anembodiment of the present disclosure. The present inventors also havefound that hexagonal shaped and hexagonal annulus patterned alignmentmark groups 1104 in FIG. 11A were immune to stress and showedsignificant growth above the hardmask.

FIG. 12A shows a top plan view of a hexagonal annulus pattern that maybe formed on a semiconductor substrate according to an embodiment of thepresent disclosure. In one embodiment, the hexagonal annulus patternincludes an array of substantially hexagonal alignment marks. Thealignment marks illustrated in FIG. 12A can be formed by overlying apatterned photoresist on a hardmask layer, the patterned photoresist mayinclude a plurality of openings having a plurality of first openingscorresponding to the array of hexagonal alignment marks in an alignmentmark region and a plurality of second openings corresponding to an arrayof semiconductor fins disposed in a device region. A lithography processis then performed to generate the array of hexagonal shaped openings(hexagonal annulus openings) in the hardmask in the alignment markregion and a fin array in the device region, as shown in FIGS. 2A and2B. The hardmask is used as a mask to etch the underlying semiconductorsubstrate to form trenches in the alignment mark region and in thedevice region. In one embodiment, each side of an hexagonal annulusopening has a width of about 0.7 μm and a length of about 11.5 μm. Inone embodiment, each opening in the device region has a width of 1.8 μmand a length of 25 μm. An epitaxial regrowth process of a semiconductorlayer is then performed in the trenches to form a protrusion portion ofthe semiconductor layer extending to a predetermined height over anupper surface of the semiconductor substrate. In an embodiment, thehexagonal edges are aligned parallel to the intersection of the c-planeand the m-planes of the substrate (i.e., along [0100], and [0010]directions). The epitaxial regrowth process is self-limiting asdescribed in FIG. 2D. In one embodiment, the epitaxial regrowth processmay be a p-GaN regrowth. In one embodiment, the p-GaN regrowth may be anMOVPE process.

FIG. 12B shows an enlarged portion of the hexagonal annulus pattern inFIG. 12A. FIG. 12C is an oblique view SEM image of an enlarged portionof the hexagonal annulus pattern in FIG. 12A according to an embodimentof the present disclosure. It is noted that the hexagonal annuluspatterned alignment marks have a significant growth above the hardmask(dark background) and good triangular-shaped sides. As illustrated, inFIG. 12C, the growth of the alignment marks proceeds along sixdirections aligned with the m-planes of the hexagonal crystal structurethat bound the growth. Thus, the outer surfaces of the hexagonal annuluspattern (i.e., the growth surfaces) define six directions, enabling thehexagonal annulus pattern to be referred to as a multi-directionalalignment mark, for example, at least three-directional growth surfaces.For example, the outer surfaces of the hexagonal annulus comprise(10-11), (01-11), (−1101), (−1011), (0-111) and (1-101) planes. Itshould be noted that the interior surfaces of the hexagonal annuluspattern are aligned with corresponding outer surfaces.

FIG. 13A shows an array of hexagonal pyramids that can be used asfiducials (alignment marks) according to an embodiment of the presentdisclosure. FIG. 13B shows an enlarged portion of the array of hexagonalpyramids in FIG. 13A. Issues in the lithography process may contributeto the irregular sizes of the pyramids. The regrowth of the hexagonalpyramids is self-limiting and substantially stops when the six sidefaces of a hexagonal pyramid intersect one another. Thus, embodiments ofthe present invention provide three-dimensional and six-directionalalignment marks that are defined by a three-dimensional shape formed bythe six side faces of the hexagonal pyramid, which each have a differentgrowth direction. In an embodiment, the six side faces of the hexagonalpyramid are the (10-11), (01-11), (−1101), (−1011), (0-111) and (1-101)planes.

The above described embodiments have a vertical regrowth semiconductorlayer both in the alignment mark region and in the device (active)region. The alignment mark region includes an array of alignment marksthat may include patterns shown in FIG. 10. The device or active regionmay include a plurality of semiconductor fins arranged in parallel toeach other. The alignment marks and the semiconductor fins can bearranged in the same direction or in different directions. In someembodiments, the alignment marks can be placed in a periphery of a wafer(substrate), in a periphery of device boundaries, or in a scribe line ofa wafer (substrate). The regrown portion of the semiconductor layer inthe alignment mark region protrudes over the upper surface of thehardmask layer. In one embodiment, the regrown portion of thesemiconductor layer in the device region has an upper surface that issubstantially flush with a bottom surface of the hardmask layer, i.e.,substantially flush with an upper surface of the substrate when thehardmask layer is removed. In one embodiment, the surface of thesubstrate is in the c-plane, and the regrowth portion of thesemiconductor layer is oriented along the m-plane directions (i.e., theedge of the regrowth where it intersects the c-plane is in one of the[1000], [0100], or [0010] directions).

Many possible alignment mark patterns may be utilized, as shown in FIGS.10 and 11A. In one embodiment, the alignment marks may include an arrayof hexagonal annulus patterns as shown in FIG. 12A. In one embodiment,each side of a hexagonal annulus pattern is aligned with the m-plane ofthe substrate. In one embodiment, the alignment marks may include anarray of hexagonal pattern as shown in FIG. 13A. In one embodiment, eachside of the bottom surface of a hexagonal pattern is aligned with them-plane of the substrate. In one embodiment, the substrate is a III-Vcompound substrate. In one embodiment, the substrate include GaN.

In one embodiment, the protruding portion of the semiconductor layer inthe alignment region is self-limiting and has a thickness of heightH=(√3/2)*W, where W is a width of a trench opening defined by thehardmask layer.

In one embodiment, the protrusion portion of the semiconductor layer inthe alignment region is an isosceles triangle or a trapezoid having abase angle in a range between 58 degrees and 65 degrees in across-sectional view.

In one embodiment, the alignment mark region has a plurality of trencheseach having a depth of about 0.8 μm, a width of about 0.7 μm, and awidth of the hardmask layer between two adjacent openings of about 3 μm.

In some embodiments, the ratio between the width of an opening and thewidth of the hardmask layer in the alignment mark region is less thanone, while the ratio between the width of an opening and the width ofthe hardmask layer in the device region is much greater than one. In oneembodiment, the ratio between the width of an opening for epitaxialgrowth of alignment marks and the width of the hardmask layer disposedbetween two adjacent openings is about 0.233. In one embodiment, theratio between the width of an opening for epitaxial growth of planarepitaxial regions in the device region and the width of the hardmasklayer disposed between two adjacent openings is between 6 and 10, e.g.,about 9. In one embodiment, the openings each may include a hexagonalannulus pattern. In one embodiment, each side of the hexagonal annuluspattern is aligned with an m-plane of the III-V (e.g., III-nitride)compound substrate.

In one embodiment, the hardmask layer in the device or active regionincludes an array of elongated openings arranged in parallel to eachother configured to form a plurality of semiconductor fins. In oneembodiment, the elongated openings each have a width in a range betweenabout 0.2 μm and about 0.3 μm, a length in a range between about 25 μmand about 1000 μm, and a pitch between two adjacent elongated openingsis in a range between about 1.9 μm and about 10 μm. The scope of thepresent disclosure is not limited to the arrangement of the fins in thedrawings in relation to the alignment marks.

In one embodiment, the substrate is a III-V compound substrate includingan n-GaN epitaxial layer, and the semiconductor layer include a p-GaNregrown epitaxial layer.

The embodiments disclosed herein are not to be limited in scope by thespecific embodiments described herein. Various modifications of theembodiments of the present invention, in addition to those describedherein, will be apparent to those of ordinary skill in the art from theforegoing description and accompanying drawings. Further, although someof the embodiments of the present invention have been described in thecontext of a particular implementation in a particular environment for aparticular purpose, those of ordinary skill in the art will recognizethat its usefulness is not limited thereto and that the embodiments ofthe present invention can be beneficially implemented in any number ofenvironments for any number of purposes.

What is claimed is:
 1. A method of forming alignment marks, the methodcomprising: providing a III-V compound substrate having a device regionand an alignment mark region, wherein the III-V compound substrate ischaracterized by a processing surface; forming a hardmask layer having afirst set of openings on the alignment mark region exposing a firstsurface portion of the processing surface of the III-V compoundsubstrate and a second set of openings on the device region exposing asecond surface portion of the processing surface of the III-V compoundsubstrate; etching the first surface portion and the second surfaceportion of the III-V compound substrate using the hardmask layer as amask to form a plurality of trenches; and epitaxially regrowing asemiconductor layer in the trenches to form the alignment marksextending to a predetermined height over the processing surface of theIII-V compound substrate.
 2. The method of claim 1, wherein the III-Vcompound substrate comprises GaN.
 3. The method of claim 1, wherein thetrenches each have a depth of about 0.8 μm.
 4. The method of claim 1,wherein the trenches each have a width of about 0.7 μm, and a width ofthe hardmask layer between two adjacent openings is about 3 μm.
 5. Themethod of claim 1, wherein a ratio of a width of an opening and a widthof the hardmask layer between two adjacent openings is about 0.233. 6.The method of claim 1, wherein the first set of openings comprises anarray of hexagonal annulus patterns.
 7. The method of claim 6, whereineach side of a hexagonal annulus pattern is aligned with an m-plane ofthe III-V compound substrate.
 8. The method of claim 1, wherein thepredetermined height is determined using an equation:H=(√{square root over (3)}/2)*W, wherein H is the predetermined height,and W is a width of a trench opening.
 9. The method of claim 1, whereinthe alignment marks are characterized by an isosceles triangular shapehaving a base angle in a range between 58 degrees and 65 degrees in across-sectional view.
 10. The method of claim 1, wherein the second setof openings comprise an array of elongated openings arranged in parallelto each other configured to form a plurality of semiconductor fins. 11.The method of claim 10, wherein the elongated openings each have a widthin a range between about 0.2 μm and about 0.3 μm, a length in a rangebetween about 100 μm and about 1000 μm, and a pitch between two adjacentelongated openings is in a range between about 1.9 μm and about 10 μm.12. The method of claim 1, wherein the III-V compound substratecomprises an n-GaN epitaxial layer, and the semiconductor layercomprises a p-GaN epitaxial layer.
 13. The method of claim 1, whereinepitaxially regrowing the semiconductor layer is self-limiting in thealignment mark region.
 14. A semiconductor device comprising: a III-Vcompound substrate comprising a device region and an alignment markregion including a plurality of trenches; and a plurality ofthree-dimensional alignment marks, each of the plurality ofthree-dimensional alignment marks being disposed in one of the pluralityof trenches in the alignment mark region, each of the plurality ofthree-dimensional alignment marks comprising a protrusion portionextending over a surface of the III-V compound substrate.
 15. Thesemiconductor device of claim 14, wherein each of the three-dimensionalalignment marks comprises a hexagonal shaped, annular, alignment mark inplan view having a predetermined height and characterized by at leastthree directional growth surfaces.
 16. The semiconductor device of claim15, wherein the protrusion portion has an isosceles triangle ortrapezoid shape with a base angle in a range between 58 degrees and 65degrees in a cross-sectional view.
 17. The semiconductor device of claim15, wherein the protrusion portion has a shape of an isosceles triangleor a trapezoid with a base substantially flush with a surface of theIII-V compound substrate in a cross-sectional view.
 18. Thesemiconductor device of claim 17, wherein the protrusion portion has ashape of an isosceles triangle and the predetermined height isdetermined using an equation:H=(√{square root over (3)}/2)*W, wherein H is the predetermined height,and W is a width of the base of the isosceles triangle.
 19. Thesemiconductor device of claim 14, wherein the III-V compound substratecomprises an N—GaN epitaxial layer, and the plurality ofthree-dimensional alignment marks comprise a p-GaN epitaxial layer. 20.The semiconductor device of claim 14, wherein the device regioncomprises a plurality of semiconductor fins arranged in parallel to eachother, wherein a regrown layer between each of the plurality ofsemiconductor fins has a regrowth surface lower than the protrusionportion of each of the plurality of three-dimensional alignment marks.